1. Field of the Invention
The present invention relates to an ultrasonic motor.
2. Description of the Background Art
Piezoelectric elements made of such as PZT (Lead Zirconate Titanate) can store large mechanical energy per unit volume, and small-sized high-power actuators using the same have been put to practical use. These are generally called as ultrasonic motors. In the following, a basic operation of a resonance ultrasonic motor will be described.
Ultrasonic motors of a type obtaining an elliptical motion by exciting a plurality of vibration modes having an identical resonance frequency in a piezoelectric element and moving a driven body by friction have been under study for a long time. For example, FIG. 10 shows an ultrasonic motor described in Japanese Patent Laying-Open No. 2007-106393. FIG. 11 shows details of an ultrasonic vibrator 121 disclosed in Japanese Patent Laying-Open No. 2007-106393.
Ultrasonic vibrator 121 has a vertically mirror-symmetric structure formed by sandwiching a reinforcing stainless plate 1211 between two piezoelectric elements 1212 and 1213. Ultrasonic vibrator 121 in the shape of a rectangular flat plate has substantially identical resonance frequencies for a first expansion/contraction vibration mode in an in-plane direction shown in FIG. 12 and a second bending vibration mode in an in-plane direction shown in FIG. 13.
On piezoelectric elements 1212 and 1213, electrodes 1216 and 1217 each divided into four are arranged. Electrodes located at opposing corners of electrodes 1216 and 1217 are respectively connected by wires. Alternating voltages φA, φB having phases 90° different from each other are applied to these two sets of electrodes, respectively.
Thus, the two vibration modes described above are excited by alternating voltages φA, φB with the phases being shifted by 90°, in the order of A, B, C, and D shown in FIG. 14. Thereby, an elliptical motion is generated at a tip portion of ultrasonic vibrator 121. If the tip portion is pressed against a driven body, the driven body is moved by a frictional force. This pressing force is generally referred to as a “preload”.
As a technique for applying the preload, Japanese Patent Laying-Open No. 2007-106393 proposes a configuration of holding ultrasonic vibrator 121 using a support projection 1214 that rotates about a shaft 127, and pulling a pulled portion 1224 provided on an opposite side surface thereof by an elastic body 129 wrapped around a pole 112, as shown in FIG. 10. Further, a ceramic contact portion 1215 is provided at a point of stainless plate 1211, and a sector-shaped rotor 122 is provided to a rotor shaft 124 via a bearing 123 to be rotatable only about an axis of rotor shaft 124.
Furthermore, an internal contact type ultrasonic motor is disclosed in a document described below, and FIG. 15 shows a structure thereof.
Yusuke Matsunaga, et al., “An In-wheel Type Micro Ultrasonic Motor Using Sector-shaped Piezoelectric Ceramics Vibrators”, issued by the Thesis Committee for the Symposium on the Basics and Applications of Ultrasonic Electronics, Proceedings of the “27th Symposium on the Basics and Applications of Ultrasonic Electronics”, Nov. 15, 2006, pp. 489-490.
Provided is a four-point contact type ultrasonic motor using two ultrasonic vibrators 21. The ultrasonic motor has a configuration in which point portions of ultrasonic vibrators 21 and 22 having the same configuration as that described above are brought into internal contact with a cylindrical rotor 1 under a preload by means of a central pantograph type preload mechanism 3.
As shown in FIG. 16, each point P has an elliptical motion trajectory, and thereby drive forces exerted in the same direction can be obtained at all points P. Further, a four-point contact type ultrasonic motor using sector-shaped ultrasonic vibrators that utilizes similar vibration modes is described in a document described below.
Takefumi Kanda, et al., “A Micro Ultrasonic Motor Using Sector-shaped Piezoelectric Vibrators and a Low-profile Preload Mechanism”, issued by the Japan Society of Mechanical Engineers, Robotics and Mechatronics Division, Proceedings of the “2007 JSME Conference on Robotics and Mechatronics”, May 10, 2007, pp. 1A2-B01.
Since the ultrasonic vibrators are pressed against a driven body from an inner side in these ultrasonic motors, they have advantages described below, when compared with a motor of a type driving an ultrasonic vibrator from an outside of a rotor as represented by the invention described above.
Since an ultrasonic motor drives a driven body by a frictional force, it is necessary to increase a preload in order to obtain a large drive torque. Due to the operation principle of the ultrasonic motor, however, if an excessive preload is applied, an ultrasonic vibrator is brought into contact with the driven body in the entire circumference of an elliptical motion, resulting in a reduction in drive efficiency. Therefore, the applicable preload has an upper limit, and this determines an upper limit of the torque. Further, a large preload increases wear in a contact point region and reduces the life of the motor.
However, according to a four-point internal contact type ultrasonic motor as shown in FIG. 16, friction forces can be obtained at four points. Further, the same driven body can obtain up to four times greater torque with the same preload, as the torque of the motor is equal to the sum of these friction forces. Conversely, only one-fourth of the preload is required to apply the same torque, suppressing wear in a contact point region and leading to an increase in the life of the motor.
Further, since ultrasonic vibrators are arranged inside a rotor, an installation area can be reduced when a rotor having the same diameter is used. Furthermore, since the ultrasonic vibrators and the rotor are held by a common component, the number of parts can be reduced.
In an ultrasonic motor shown in FIG. 17, sector-shaped ultrasonic vibrators are used. Although vibration modes used are basically similar, one axis of the elliptical motion at contact points between rotor 1 and ultrasonic vibrators 21a, 22a is in the direction of the tangent to rotor 1. Therefore, a large amplitude can be obtained and drive efficiency can be improved, when compared with a case where rectangular ultrasonic vibrators are used.
However, actual fabrication of an ultrasonic motor always involves a machining error. It is difficult to establish contact at four points in the configuration of FIG. 16, resulting in contact at three points, that is, both ends of one ultrasonic vibrator and one end of the other ultrasonic vibrator. Thereby, there occurs a problem that desired performance cannot be exhibited.
A specific description will be given below. It is to be noted that, in the description and drawings below, for the sake of clarity, the description will be given in a two-dimensional plane, and the shape of the ultrasonic vibrators is shown only in outline. Although the description will be given on rectangular-shaped ultrasonic vibrators, the same description applies to sector-shaped ultrasonic vibrators as described above. Further, regarding a mechanism holding the ultrasonic vibrators and pressing them against a rotor, only its function is to be considered, and thus it is abstractly shown using a mark representing a piston.
A description will be given with reference to FIG. 18. Two ultrasonic vibrators 21 and 22 are used to drive rotor 1, which is completely round. The two ultrasonic vibrators 21 and 22 are brought into internal contact with rotor 1 by means of pantograph type preload mechanism 3 interposed therebetween.
As shown in FIG. 18, left ultrasonic vibrator 21 in the drawing is in internal contact with rotor 1. The distance from right ultrasonic vibrator 22 to left ultrasonic vibrator 21 can be changed by preload mechanism 3. Therefore, right ultrasonic vibrator 22 can also be brought into internal contact with rotor 1 at one point. The other ultrasonic vibrator 21 is also in internal contact with rotor 1. If ultrasonic vibrators 21 and 22 ideally have the shape of a rectangle and are arranged in parallel, four-point contact can be established.
Actually, however, as shown in FIG. 19, a machining error in the shape of the ultrasonic vibrators is unavoidable, and the shape of the ultrasonic vibrators is not exactly rectangular in most cases. Further, a deviation in the shape of the ultrasonic vibrators may also occur due to subsequent events such as wear of a contact point.
As a result, as shown in FIG. 19, only one point is in contact with rotor 1, and the other point is away from rotor 1 by a gap d shown in FIG. 19. Therefore, if there occurs a large positional deviation due to the machining error described above, the latter point cannot come into contact with rotor 1 even though it performs the elliptical motion, and thus a desired frictional force may not be able to be obtained.
To solve the above problem, the machining error should be suppressed to allow gap d to be kept in a range smaller than the amplitude of the elliptical motion at the contact point of ultrasonic vibrator 22 described above. Generally, however, the amplitude of the elliptical motion is on the order of submicrons, and the machining accuracy is required to be further smaller than the amplitude. Such an accuracy is in a range that is extremely difficult to achieve with an ordinary processing method such as machining.
As described above, a torque that is only up to three-fourth of a desired torque can be obtained in the ultrasonic motor with the above configuration. Further, regarding the ultrasonic vibrator having only one point in contact with the rotor, a preload is actually concentrated on the one contact point, and the preload is double the preload applied to the opposing ultrasonic vibrator. Therefore, there is concern that the torque and the wear might be further worsened than the desired values described above.